For the laying of cables and pipes in the ground small tunnels of diameters in the area of about 45 to 200 mm are needed in front gardens, under streets, railways, waterways and established surfaces, which tunnels are often made by means of percussive or impact boring devices. The displacement heads of customary devices of this type can exhibit quite good displacement and crushing characteristics resulting from special head shapes. They however are not controllable in their forward direction, so that in the case of simple devices of this type no possibility exists for controlling them around a curve or to correct for their deviation from the desired path.
To avoid these disadvantages different solutions have already been proposed. So it is known from DE-A-39 11 467 to provide an inclination on the forward side of the earth boring device. This inclination presses the head of the device toward one side upon penetrating the ground. To achieve a straight run such a head has to be continuously rotated about its longitudinal axis. Thereby the inclination during the forward movement of the device presses the device in sequence upwardly, to the right, downwardly and to the left, so that in the event of a uniform rotation of the head the device as a result moves in a straight line. A head with such an inclination has indeed no longer the necessary crushing and displacement characteristics making possible forward movement in inhomogeneous and firm types of ground.
To reduce the resistance at the point of the device head and to ease the forward movement of the device in certain cohesive and sandy types of ground it has already been proposed to break up or entirely wash away the ground directly in front of the device head by a strong fluid jet. In keeping with this, in US-A-4 674 579 an earth boring device is known which is asymmetric in the front and has an outlet for a fluid jet directed from one side and inclined to the axis of the device. Since the ground in the area of the fluid jet is deflected or washed away the ground in this area presents the least resistance to the earth boring device. Upon its forward drive the earth boring device therefore deflects in the direction of the fluid stream. Thereby it influences the forward movement direction of the device. For straight movement this bore head much like that of the above described device with an inclined head has to be continuously rotated. A disadvantage of this solution is that the mechanism required for rotating the head does not stand up long against the rough drive conditions. This is especially true for so-called ram boring devices wherein the dynamic hammer impacts needed for the advancement of the earth boring device are so strong that the elements for guiding and rotating the head become worn out in short time.
The same disadvantages are present in the earth boring device with symmetrical head known from US-A-4 714 118, on which several asymmetrically arranged jet nozzles are provided. These nozzles are either all opened or all closed. For straight path movement the bore head must again be rotated.
An earth borer is known from US-A-3 365 007 in the boring head of which nozzles are formed for the discharge of boring fluid. The fluid discharge through these nozzles can be controlled. Since the boring head for penetration into the ground must be rotated it must be stopped to change the boring direction. After the stopping of the boring head by controlling certain nozzles the ground is washed away in the desired place. When the borer is again driven, it moves into the washed out space and thereby deviates in the desired direction from its previous boring direction. A change of direction during the forward drive is in this case, however, not possible.
An earth borer according to US-A-3 746 108 also has the same disadvantage. It differs from the known solution of US-A-3 365 007 in that the jet nozzles on the boring head are arranged asymmetrically and are not selectively controllable. To be able to change the boring direction, again the boring head must be stopped and so oriented that the nozzles point in the desired direction. After the washing away of a lateral space the drive of the borer can again be resumed.